Aryl hydrocarbon receptor (AhR) is a drug-sensing receptor activated by environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and is known to drive regulation of target genes in various human cell types. Its involvement in TCDD-mediated regulation of target genes in human hepatocytes however remains to be formally demonstrated. To gain insights into this point, we have analyzed the effects of AhR silencing on the regulation of various genes targeted by TCDD in primary human hepatocytes and highly-differentiated human hepatoma HepaRG cells. Efficient AhR knocking-down was performed through dimethyl sulfoxide-based transfection of small-interfering RNAs targeting AhR (siAhR). SiAhR-transfected human hepatocytes or HepaRG cells, exposed to TCDD, were found to exhibit reduced mRNA expression of various TCDD-responsive genes, i.e. CYP1A1, CYP1A2, CYP1B1, ALDH3A1, IL17RB, FER1L3 and SLC7A5, when compared to TCDD-treated counterparts transfected with non-targeting small-interfering RNAs. AhR silencing was moreover shown to markedly counteract TCDD-mediated induction of CYP1A1/CYP1A2/CYP1B1-related ethoxyresorufin O-deethylase activity in both human hepatocytes and HepaRG cells. It also concomitantly decreased constitutive mRNA expression of some target genes such as CYP1A1, CYP1A2, CYP1B1 and ALDH3A1. Taken together, these data indicate that AhR plays a crucial role in both basal and TCDD-induced expression of target genes in human hepatocytes.
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